* lisp/nxml: Use standard completion; it also works for company-mode

* lisp/nxml: Use standard completion; it also works for company-mode

* lisp/nxml/nxml-mode.el (nxml-complete): Obsolete.
(nxml-completion-at-point-function): Remove.
(nxml-mode): Don't set completion-at-point-functions.
* lisp/nxml/rng-nxml.el (rng-nxml-mode-init): Set it here instead.
(rng-completion-at-point): Rename from rng-complete and mark it
non-interactive.  It is now to be used as completion-at-point-function.
(rng-complete-tag, rng-complete-end-tag, rng-complete-attribute-name)
(rng-complete-attribute-value): Don't perform completion, but return
completion data instead.
(rng-complete-qname-function, rng-generate-qname-list): Add a few
arguments, previously passed via dynamic coping.
(rng-strings-to-completion-table): Rename from
rng-strings-to-completion-alist.  Don't return an alist.  Don't both
sorting and uniquifying.

* lisp/nxml/rng-util.el (rng-complete-before-point): Delete function.
(rng-completion-exact-p, rng-quote-string): Delete functions.

* lisp/nxml/rng-valid.el (rng-recover-start-tag-open)
(rng-missing-attributes-message, rng-missing-element-message)
(rng-mark-missing-end-tags): Use explicit ".." in formats rather than
calling rng-quote-string everywhere.